6 Replica management problems Replica states must be kept mutually consistent Replica servers must propagate updates very quickly Hard to do without affecting performance particularly, when routing and bandwidth allocation fall outside the designer control, as in current Internet Real, or virtual, partitions in the Internet use of ping allows one to suspect failures -> sites hosting replicas may get inconsistent views of who is up and who is down -> inconsistent update propagation 11 Maintaining replica consistency Strict consistency: update must be propagated from replica server to peer servers in real-time Pros: client always provided with up-to-date information Cons: low performance update operation does not scale Loose consistency: updates propagated in the background Pros: scaleable Consistency among replicas cannot be guaranteed Current techniques of atomic update/multicasting do not provide viable solution, as yet assume asynchronous communications do not scale Multilevel solution: J. Gray, P. Helland, P. O'Neil and D. Shasha, "The dangers of replication and a solution", ACM SIGMOD Record, 25 (2), pp , June

7 Dynamic binding problems Early Web design decision: one-to-one mapping NAME <-> SINGLE COPY of resource i.e., URL contains Internet host name of server machine and name of resource on server Difficult to distribute load across replicas without augmenting browser s capabilities we are looking for solutions that can be implemented using commercial components 13 Dynamic Binding: current solutions Clustering Monitoring Mirroring HTTP REDIRECT DNS round-robin CISCO distributed director QoS based (our approach) Common features: aim to achieving scaleability and responsiveness by augmenting processing power of a site meet objective only partially: may succeed in optimizing service throughput cannot guarantee timeliness as Internet latency is beyond their control 14

9 CISCO local director Commercial router implementing NAT intelligent redirection of pkts by monitoring host servers response times, so as to perform load distribution and maximize service throughput if a host fails, its response time = -> that host receives no requests until back to service Pros can perform load distribution/balancing does not affect (i.e. increase) Internet traffic Cons router: single point of failure no control over Internet latency requires ad hoc techniques to redistribute load of faulty servers 17 Monitoring Decentralized approach network s/w of server A configured to respond to both its own IP address and that of a server B A monitors B, via network level ping messages if B fails, A takes over, and responds to requests directed to B 2 machine cluster can be engineered so that A monitors B, and viceversa, in order to tolerate a single machine failure Pros support for dynamic binding does not rely on router Cons servers must be connected by redundant, high bandwidth links run RT OSs so that network protocol processing experience bounded scheduling delays, only no control over Internet latency 18

10 Data distribution: physical partitioning service LAN Web Server 1 Web Server 2 Web Server 3 Web Server 4 Shared SCSI BUS data physically partitioned among Web Servers normal operation: each server has exclusive access to its own primary disc (e.g., WS1 <-> D.1) Server crash: use of monitoring technique each server can take over work of another server in the event of crash, and access crashed server storage via SCSI bus 19 Data distribution: physical partitioning Resource set associated to particular machine (i.e., resource name includes host name ) Pros: ok for both read-only and read-write operations as resources are not replicated no mechanisms are required to maintain data consistency within cluster no special techniques for load distribution required Cons: unpredictability of load distribution most popular server may receive disproportionate load re-distribution of load (i.e. migration of resources between machines) may corrupt intra-site links 20

12 Mirroring Mirroring site replica servers geographically distributed client requested to indicate its geographical location, or select manually preference from a list of URLs usually client selects geographically closer URL, or one wich is reported to have higher performance e.g. EU - NoEs Information Service list of mirror site URLs estimate of time required to transfer a document from each mirror Pros: easy to provide Cons: static (no load balancing, no dinamic binding, no contol over communication delays) 23 HTTP REDIRECT Client connects to main server Server replies with HTTP REDIRECT command to route request to one of N available replica servers Client makes subsequent requests to that server Pros: easy to implement (automatic version of mirroring) can provide dynamic binding Cons makes visible to clients URLs of the replica servers can be stored in hot lists by clients, or search engines, thus defeating subsequent load balancing 24

13 DNS DNS round-robin allows mapping of a host name to one of several IP addresses resolves host name in one address, in round-robin (rr) -> 2 consecutive requests get 2 different addresses Internet Host_id IP address www1.cs.unibo.it www2. cs.unibo.it www3. cs.unibo.it Principal problems service availability in the event of host failures DNS not designed to propagate updates throughout the system quickly Server crash: not possible to update the whole DNS timely -> clients my continue directing their requests to crashed servers until they get service not available hierarchical organization of DNS: may defeat load balancing via rr 25 Hierarchical organization of the DNS Browser interrogates local DNS. If this cannot resolve name, it interrogates higher level DNS. Process continues until name can be resolved by primary DNS. Primary performs round-robin and returns response along the path, to the browser DNSs in path cache responses Next request from browser can be resolved by DNS in path as host name <-> IP address mapping is in its cache Round-robin by primary is bypassed may cause disproportionate load to one server Remedy: TTL associated to DNS cache entry entry removed from cache after TTL expires Problem of TTL tuning the shorter the TTL, the greater the load on the primary DNS Examples: HP main site TTL = 10, Microsoft TTL = 2 hrs. in general TTL = 24 hrs (for not frequently changing info) 26

14 CISCO DISTRIBUTED DIRECTOR selects optimum server for a particular client via use of routing info. inherent in network optimum server found deploying proprietary protocol to query s/w agents running on gateway closest to each server query contains client address agents calculate n. of hops between client and server DD selects server closest to client can operate as DNS acts as primary DNS responds to requests returning a single server address or as HTTP redirector acts as a Web server accepts requests and returns HTTP code temporarily moved to redirect clients to approriate server requires agent s/w running at each site and CISCO gateway systems! 27 QoS-based strategy Assumption: Web replica servers geographically distributed across the Internet (as in fig. before) Clients get bound to their relative most convenient replica server most convenient: replica server providing the client with the shortest URT URT: elapsed time between the generation of a browser request and the rendering of that request to the invoking browser 28

20 C 2 LD approach (fragment downloading) C 2 LD splits the browser s requests into more sub requests for fragments. When a fragment is received from a replica server, another fragment is requested to that server web replica 1 page fragment 2 web replica 2 page fragment n fragment 1 reassembled page web replica n page fragment m fragment 3 browser site 39 C 2 LD analytical model (1) Monitoring period S: C 2 LD expects to receive the fragments at the end of S given i: index of the replica server r: index of the sub request to replica i T: instant in which C 2 LD receives response for the sub request r-1, from i Si,r* time left before the end of the current monitoring period S at instant T, C 2 LD requires a new fragment to replica i, and expect to receive response at the end of current period S 40

White Paper Overview Many enterprises attempt to scale Web and network capacity by deploying additional servers and increased infrastructure at a single location, but centralized architectures are subject

1. Comments on reviews a. Need to avoid just summarizing web page asks you for: i. A one or two sentence summary of the paper ii. A description of the problem they were trying to solve iii. A summary of

1 First Midterm for ECE374 03/24/11 Solution!! Note: In all written assignments, please show as much of your work as you can. Even if you get a wrong answer, you can get partial credit if you show your

1. Definitions The following definitions apply, in addition to those in the General Terms and Conditions and the General Services Schedule. ARP means Address Resolution Protocol. Border Gateway Protocol

Cisco Application Networking for IBM WebSphere Faster Downloads and Site Navigation, Less Bandwidth and Server Processing, and Greater Availability for Global Deployments What You Will Learn To address

Principles and characteristics of distributed systems and environments Definition of a distributed system Distributed system is a collection of independent computers that appears to its users as a single

EECS 489 Winter 2010 Midterm Exam Name: This is an open-book, open-resources exam. Explain or show your work for each question. Your grade will be severely deducted if you don t show your work, even if

Testing & Assuring Mobile End User Experience Before Production Neotys Agenda Introduction The challenges Best practices NeoLoad mobile capabilities Mobile devices are used more and more At Home In 2014,

MEASURING WORKLOAD PERFORMANCE IS THE INFRASTRUCTURE A PROBLEM? Ashutosh Shinde Performance Architect ashutosh_shinde@hotmail.com Validating if the workload generated by the load generating tools is applied

Topic: Load balancing 4/03/2014 (c) A. Mariën 1 High Availability Key property: availability high is relative Important to determine correctly Cost of unavailability Business continuity: how long can one

Department of Computer Science Institute for System Architecture, Chair for Computer Networks Caching, Content Distribution and Load Balancing Motivation Which optimization means do exist? Where should

The fundamentals of TCP/IP networking TCP/IP (Transmission Control Protocol / Internet Protocols) is a set of networking protocols that is used for communication on the Internet and on many other networks.

CHAPTER 3 Using IPM to Measure Network Performance This chapter provides details on using IPM to measure latency, jitter, availability, packet loss, and errors. It includes the following sections: Measuring

Requirements of Voice in an IP Internetwork Real-Time Voice in a Best-Effort IP Internetwork This topic lists problems associated with implementation of real-time voice traffic in a best-effort IP internetwork.

System i Networking TCP/IP routing and workload balancing Version 5 Release 4 System i Networking TCP/IP routing and workload balancing Version 5 Release 4 Note Before using this information and the product

Key Components of WAN Optimization Controller Functionality Introduction and Goals One of the key challenges facing IT organizations relative to application and service delivery is ensuring that the applications

Smart Tips Enabling WAN Load Balancing Overview Many small businesses today use broadband links such as DSL or Cable, favoring them over the traditional link such as T1/E1 or leased lines because of the

Overlay Networks An overlay is a logical network on top of the physical network Routing Overlays The simplest kind of overlay Virtual Private Networks (VPN), supported by the routers If no router support

Performance Evaluation of Linux Bridge James T. Yu School of Computer Science, Telecommunications, and Information System (CTI) DePaul University ABSTRACT This paper studies a unique network feature, Ethernet

MULTI WAN TECHNICAL OVERVIEW The Multi WAN feature will allow the service provider to load balanced all client TCP and UDP traffic only. It also provides redundancy for HA. Traffic that is load balanced:

This chapter describes the different networking topologies supported for this product, including the advantages and disadvantages of each. Select the one that best meets your needs and your network deployment.